Traveling-wave tube



Oct. 29, 1957 G. R. BREWER ErAl. 2,811,663

TRAvELmG-WAVE TUBE Filed oct. 22, 1954 Era-.2'.

2,81 1,663 Patented Oct. 29, 1957 TRAVELlNG-WAVE TUBE George R. Brewer, Palos Verdes, and Horace R. Johnson and Charles K. Birdsall, Venice, Calif., assignors to Hughes Aircraft Company, a corporation of Delaware Application October 22, 1954, Serial No. 463,902

Claims. (Cl. 315-3.5)

This invention relates to microwave tubes and more particularly to means for focusing a hollow electron stream of a traveling-wave tube.

A type of traveling-wave tube oscillator called a backward-wave oscillator is well-known. In this oscillator it is conventional practice to project a hollow electron stream contiguous to a periodic slow-wave structure to cause interaction of an electromagnetic wave propagated along the structure and this stream. A hollow instead of a solid electron stream is employed to attain optimum efficiency. Further, in order to effect maximum interaction of the stream and the wave, it is desirable that the stream be projected as close to the slow-wave structure as it is physically possible. However, an appreciable number of electrons collide with the slow-wave structure whereby the eliiciency of the oscillator is materially reduced. In order to produce collimation of electrons in the hollow stream, a solenoid is normally positioned concentrically about the stream to produce an axial magnetic field which constrains or confines the stream.

The employment of a solenoid in a traveling-wave tube is one of the outstanding disadvantages of the backwardwave oscillator in that the solenoid is relatively bulky and heavy and requires a considerable amount of directcurrent power.

Furthermore, a hollow electron stream cannot be accurately focused simply with the use of a system of magnetic lenses of the conventional type because the inner surface of the focused stream tends to collapse, so that a relatively large number of electrons on the inner boundary surface of the hollow stream have trajectories which prevent them from interacting with the electric fields of the electromagnetic wave. Eliiciency is consequently reduced still further.

An object of the invention is therefore to provide means whereby a hollow electron stream may be properly focused through substantially its entire length.

Another object of the invention is to provide an improved focusing system for the hollow electron stream of a traveling-wave tube oscillator whereby the stream may be focused through substantially the complete length of the tube.

In accordance with the present invention a plurality of magnetic electron lenses are provided at periodic intervals along a hollow electron stream, the direction of the axial magnetic field of the lenses being in one direction within a predetermined radius and being in the opposite direction outside of that predetermined radius. The lens elds and the beam perveance are then adjusted to balance the inward magnetic force against the outward centrifugal and space charge forces of the electrons. The outer boundary surface of the hollow stream is then focused toward a point between the lenses. The magnetic flux density produced at the predetermined radius is equal to zero in the longitudinal center of the lens and the hollow stream is introduced into the field of the first electron lens in a manner to cause the inner boundary surface of the hollow stream to be focused outward toward the same points as the outer boundary surface, the predetermined radius falling within the maximum deviation of the inner boundary surface.

The above and other objects of the invention will become apparent from the following description taken in conjunction with the accompanying drawing made a part tf this specification. In the drawing:

Fig. l is a sectional view of an oscillator embodying the present invention;

Fig. 2 is a diagrammatic sectional view of the electron stream employed with the oscillator of Fig. l; and

Figs. 3 and 4 are sectional views of alternative lens structures of the invention.

Referring to the drawing, a backward-wave oscillator is shown in Fig. 1 comprising a traveling-wave tube 10 including an evacuated envelope 12 having an enlarged portion 14 at its left extremity, as viewed in the drawing. In the enlarged envelope portion 14 an electron gun 13 is shown comprising a hollow cylindrical cathode 16 which is disposed about a longitudinal dielectric rod 18, a cathode heater 20, internal and external focusing electrodes 22 and 24, and an accelerating anode 26. Dielectric rod 18 may be made of glass or a number of dielectrics, e. g., those known commercially as synthetic sapphire and as Zircon.

Cathode heater 20 is supplied with current by a heater source of potential 28. Focusing electrodes 22 and 24 may have a frusto-conical configuration which cause a hollow, cylindrical electron stream 21 to be formed from electrons emitted by the cathode 16. Focusing electrodes 22 and 24 are maintained at a suitable potential with respect to the cathode 16 by an appropriate connection thereto. The oscillator is tuned by means of a voltage divider 30 which has an adjustable contact arm 32 to which the cathode 16 is connected. Voltage supply 34 is connected across the voltage divider 30, voltage supply 34 having its positive terminal connected to ground. Accelerating anode 26 may be grounded as shown.

Disposed coaxially within the envelope 12 in the direction of electron flow from the gun 13 there is shown a matching ferrule 36 connected over an antenna lead 38 to a conductive helix 40. A collector electrode 42 is disposed at the right end of the envelope 12 to intercept the stream electrons. In order to prevent secondary electrons from reaching the helix 40, collector 42 is maintained at a few hundred volts positive with respect to ground by a potential source 43 having its negative terminal grounded. Helix 40 with antenna lead 38 and matching ferrule 36 are al1 maintained at ground potential by a connection to accelerating ano-de 26.

At the right end of helix 40, a lossy material 44, e. g. a commercial preparation known as Aquadag, is deposited on the last few turns of the helix to prevent an undesirable type of self-oscillation. This self-oscillation may be produced by the reflection of microwaves traveling along the helix at the collector 42 and the subsequent reflection and regeneration of these signals at the electron gun 13.

The dielectric rod 18 extends through substantially the complete length of the helix 40. At periodic intervals along the rod 18, a plurality of conductive coils 46 is wound about the rod 18. The coils 46 are energized by a potential source 48, the leads extending through or along rod 18.

Outside the envelope 12, at its left end, rectangular output waveguide 50 is shown having a conductive sleeve 52 disposed about the envelope coextensive with the matching ferrule 36. Waveguide 5I] also has a conventional shorted termination 54.

A plurality of solenoid windings 56 is positioned at regular intervals along and about the envelope to focus the hollow electron stream produced by the electron gun 13. The windings 56 are provided with magnetic shields 58 and are energized by a focusing source of potential 60. The windings 56 are disposed at the same axial positions of the coil 46 on dielectric rod 18. The coils 46 may be provided with magnetic cores not shown; however, such cores may be undesirable since they reduce the axial tiux linking the cross-section of the Yelectron stream. The windings 56 in cooperation with the coils 46 are designed to produce a variation of the axial flux, qbz, having a variation with radius r, which may be of the type shown in Fig. 2 where rs represents the radial position of an electron in the outer boundary surface of the hollow electron stream; r, represents the radial position of au electron in the inner boundary surface; n,0 and fsm are the maximum and minimum radii of r respectively; ri and rim are the maximum and minimum radii of rl, respectively; and where rr is the radius at which the axial tlux density, BZ, and the ux linking the path of an electron, fpz, are both equal to zero.

In the operation of the oscillator of Fig. l, noise at the collector end of the traveling-wave tube causes a backward-wave to be propagated along the helix 40. Mutual interaction of the electron stream produced by the electron gun 13 and the backward-wave is produced by keeping the ratio Gg r,o near unity. As the stream enters the rst electron lens which is produced by the left-hand winding 56 and its associatedtcoil 46 it is introduced at L dra dr, z---2- with d'- d. where L is the distance between lenses and z, which is equal to zero at the center of the left-hand winding 56, is distance along the stream 21. The magneticv field between r'm and r1D is produced by one winding 57 which opposes the lield of an associated coil 45 in the stream. The fields produced by the windings 56 are then employed to overcome the space charge and centrifugal forces of an electron in an r, orbit. The ux linking an inner electron, at a radial distance ri, as it enters a lens will become substantial. Centrifugal force then carries it past the radius r where the field produced by a winding 56 will then refocus it. The inner electron will then have an angular velocity equal to zero when ri again equals rv. As the electron again enters the field of the succeeding coil 46, centrifugal force prevents the collapse of the inner boundary surface of the stream because the inner electron is caused to rotate.

In Fig. 3 another lens structure is shown which may be employed to give a suitable axial ux density variation with radius. Two axially polarized permanent magnets 130 and 132 are shown disposed concentrically about the hollow stream 21 with appropriately indicated magnetic poles. Both magnets 130 and 132 are hollow cylinders which may be of material having hard or permanent magnetic properties, e. g. an alloy commonly known by the trade name Alnico. Magnet 132, being the smaller of the two, is disposed concentrically within magnet 130. The external cylindrical magnet 130 is provided with two inwardly extending annular appendages or rims 131. The internal magnet 132 is likewise provided with annular appendages 133. The magnetization ofthe external magnet 130 must be greater than that of internal magnet 132 because the magnetic tiux in the external magnet 130 is diverted through the internal magnet 132. A plurality of pairs of magnets such las magnets 130 and 132 would, of course, be employed at regular intervals along the hollow stream 21 to'focus it'.

As shown in Fig. 4, in order to prevent linx leakage in the employment of magnetssuch as magnets 130 and 132, an alternative embodiment of the invention may be used. There a magnetic electron lens is produced by a cylindrical pole piece of iron or other magnetic material having inwardly extending annular appendages or rims 142 surrounding two outer solenoid windings 144 and a central winding 146, the outer windings 144 providing aiding fields and the central winding 146 providing an opposing eld. A plurality of such pole pieces and windings may again be positioned at regular intervals along the stream 21 to focus it. A radially varying magnetic field may thus be produced, the fields of the outer windings protruding into `the center of the pole piece 140.

In order to produce a satisfactory design of the electron limit of the present invention it may be necessary to solve the motion equation of an electron. In the following development the meter-kilogram-second system of units is followed throughout. Before the motion equation can be solved, the axial flux density variation, Bz, in one of the electron lenses produced by a winding 56 and an associated coil 46 must be found. It'wll generally vary as a function of axial length, z, and radius, r as B2=f1(z)f2(r) The electric field intensity Es at r, due to space charge edlen-.732) E. A 2T3b where C, is the charge and e., is the permittivity of free space of 8.854X 10-12 farad/meter.

The total force on an electron at r, is

where m is the mass of an electron or 9.12 X 10-a1 klograms, r, is the radial acceleration of an electron in the outer boundary surface of the stream, 6 is the angular velocity of the electron, Baz is the ux density at r and e is the charge of an electron or 1.59 lil-'9 coulomb. By substitution where fz(r,) contains a reference to maximum field strength Bn and radius r,...

According to Buschs theorem, if no flux links the cathode 16,

where Ip is.A the stream current and VQ is the stream Valtra@ esamina and substituting Equations 5, 6 and 7 into Equation 2 using in that Bfz=fi(z)f2(r1), fz (ri) also containing a reference to Bo and rr.

The correct lens spacing is dictated by the characteristics of the stream which it is desired to focus.

An approximate design procedure of the invention may be obtained by considering that the 'leld of a neighboring Equations 8 and 9.

on the magnetic eld distribution, e. g. such as 1 f.(z z, 1o

Zrm should be about equal to the inside diameter of the helix. For a given beam perveanee, the maximum value of ri., or rim will be determined by the maximum perveance of the hollow stream. Generally rim should not be so small as to bring the stream outside the fields of the wave propagated along the helix 40.

The only three parameters for proper design which remain undetermined are C, which is proportional to the hollow stream perveance, B., and rv. B and rv are both dependent upon C. With the use of an analogue cornputer, the trajectories of electrons having radii rs and ri may both be found by solving Equations 8 and 9 and by assuming different values for C, Bo and rv. From the Vtrajectory plots in the solution of Equation 9, Bo may be plotted as a function of r,I for a given lens spacing, say

For the same lens spacing, from the trajectory plots in the solution to Equation 8, B., may again be plotted a number of times as a function of rv for different condi- T30 if rml/fao decreases as C increases. From the other in` formation known Bo and rv may be plotted as a function of C for the conditions of focus. Having selected a value for C from the curve of nml/iso versus C, Bo and rv and therefore the field strength and distribution produced by the windings 56 and the coils 46 may be determined.

What is claimed is:

l. In an electron stream-type tube, electron stream focusing apparatus comprising an electron gun for producing a hollow electron stream and for directing it along a predetermined recti-linear path, a plurality of outer electron lens devices, an inner electron lens device disposed within each of said outer electron lens devices, each associated pair of said lens devices being disposed at periodic intervals along said predetermined path and arranged to develop a magnetic eld to focus the inner and outer electrons of the hollow stream substantially at common focusing apparatus comprising an electron gun for prothe axial magnetic flux without said predetermined radius in said planes of symmetry, and means for directing said stream along said predetermined path through said lens 7. In an electron stream-type tube, an electron stream focusing device comprising an electron gun for producing a hollow electron stream, a plurality of magnetic solenoid windings disposed at periodic intervals along a predetermined path for producing a plurality of magnetic electron lenses, a magnetic shielding member disposed about each of said solenoid windings, a plurality of magnetic coils, each being disposed conoentrically within one of said solenoid windings, whereby the axial magnetic ilux density produced within a predetermined radius is negative with respect to that produced outside of said predetermined radius, and means for directing said stream along said predetermined path between said solenoid windings and said conductive coils at a velocity to cause the electrons in the inner andV outer boundary sur-faces of said hollow stream to be focused periodically at the same axial positions along said path.

8. In a traveling-wave tube having a slow-wave structure and an electron gun for producing a hollow electron stream, an electron stream focusing device comprising a plurality of magnetic solenoid windings disposed at periodic intervals along and about the slow-wave structure for providing a plurality of magnetic electron lenses, a magnetic shielding member disposed about each of said solenoid windings, a plurality of magnetic coils, each being disposed concentrically within the slow-wave structure and one of said solenoid windings, whereby the axial magnetic flux density produced within a predetermined radius is negative with respect to that produced outside of said predetermined radius, and means for directing the hollow electron stream through said electron lenses so as to pass between the slow-wave structure and said conductive coils at a velocity to cause the inner boundary surface of the stream to vary about said predetermined radius.

9. In a traveling-wave tube having a conductive helix for propagating microwaves and an electron gun for producing a hollow electron stream, an electron stream focusing device comprising a plurality of external axially polarized magnetic cylinders disposed about the helix at periodic intervals therealong, a plurality of internal magnetic cylinders having aiding internal fields, each of said internal cylinders being disposed ooaxialiy-` between o of said external magnetic cylinders `and the helix, and! means for directing the stream through the helix and through said cylinders at a velocity to cause the electrons in the inner and outer boundary surfaces of the stream lo be focused periodically at the same axial positions along the helix,

l0. In a backward-wave oscillator having a conductive helix for propagating microwaves and an electron gun for producing a hollow electron stream, an electron stream focusing devise comprising a plurality of magnetic cylinders disposed about the helix at periodic intervals therealong, a central solenoid winding disposed within each of said magnetic cylinders outside of the helix, two ada# cent solenoid windings disposed on each side of each central winding, said central Winding producing a mag netic field in opposition to those of said adjacent wiud ings, whereby the axial magnetic linx density produced'. at. the central planes of symmetry of said magnetic cylinders Within a predetermined radius is negative with respect to that produced at said planes outside of said predetermined radius, and means for directing the hollow electron stream through the helix and through said magnetic cylinder s at a velocity to cause the inner boundary surface of the stream to vary about saidY predetermined radius.

References ited in the le of this patent UNITED STATES PATENTS Litton Dec. 22, 1942 Lindenblad Oct. 27, 1943 OTHER REFERENCES 

